A capacitance video disc system has been described by Clemens in U.S. Pat. No. 3,842,194. The disc described comprises a plastic disc containing an information track having audio, video and color information in the form of a surface relief pattern arranged in a spiral groove on the surface of the disc which can be played back with a stylus. The video disc of Clemens had a conductive metal coating to provide the conductivity required for capacitive pickup and a thin layer of a dielectric material thereover. An electrode on the playback stylus completed the capacitor.
Improvements have been made in this system whereby the disc is made from a conductive plastic material, as disclosed by Fox et al in copending application Ser. No. 818,279, filed July 25, 1977. A polyvinylchloride based molding composition is mixed with sufficient amounts of finely divided carbon black particles so that the resulting composition has the conductivity required for capacitive playback. A thin layer of the polyvinylchloride surrounds each of the conductive carbon particles so that a thin dielectric layer is present at the surface. Improvements to the conductive molding composition are described by Martin et al in copending application Ser. No. 003,363, filed Jan. 15, 1979.
The use of a conductive plastic eliminates the need for separate metal and dielectric layers on the disc surface and thus has simplified manufacture of the disc. However, we have found the stability of these discs with respect to environmental fluctuations of temperature and relative humidity has been less than satisfactory. Discs which on initial playback have excellent playback characteristics, deteriorate with respect to playback after storage under ambient conditions. This deterioration is particularly marked when the discs are exposed to high temperature and/or high relative humidity conditions.